Many modern day machines, from data printers to positioning devices to robots and the like, require an element to reciprocate at relatively high speeds and with considerable accuracy. Today's standards require such equipment to have good positional repeatability, reliability and long life. A typical element for accomplishing this is an anti-backlash nut threaded onto a lead screw that traverses back and forth when the screw rotates.
One such element is disclosed in our U.S. Pat. No. 4,131,031 which includes an anti-backlash nut assembly which undergoes translational movement back and forth along a lead screw in response to the rotation of the screw. The nut itself is divided into two halves, both of which have internal threads complementary to the thread on the screw. The two portions or halves of the nut are retained on the screw, being secured in a radial direction, by a spacer in the form of a tube or collar. A spring is used to apply an axial biasing force in the longitudinal direction of the screw, forcing the nut halves apart, to minimize or eliminate backlash.
While anti-backlash nuts made in accordance with our above-identified patent have proven satisfactory, they are somewhat limited because of the spring employed. The spring biases the nut halves apart to urge their thread flanges against opposite thread flanks of the lead screw. If a load were imparted to one of the nut halves without the spring in place, the opposite nut half would translate with respect to the first nut half because there is nothing to restrain it. The anti-backlash feature of the invention depends upon the axial force of the spring itself. Since the load is transferred through the spring, a spring of high compressibility or stiffness causes too great a frictional force and thus, a loss of efficiency. Conversely, too weak a spring would not adequately reduce backlash. Thus, the anti-backlash aspect is only as good as the force of the spring.
Another element is to be found in our U.S. Pat. No. 4,353,264 in which we have disclosed an anti-backlash nut assembly which also undergoes translational movement longitudinally along a screw in response to rotational movement of the screw. The mechanism employs a nut which is split into two nut halves which are internally threaded with the same thread and hand as the screw. The two nut halves are retained in the radial direction by a coaxial cylinder surrounding portions of the nut halves. In addition, a spacer is mounted on the retainer means between the nut surfaces and is biased against at least one of the surfaces to close any gap which may occur between the thread of the screw and the thread of the nut. Thus, the biasing is accomplished by employing a number of mechanical elements.
Whenever a number of elements are used for establishing this biasing, they are inherently weak in terms of axial stiffness due to mechanical and manufacturing fit problems. The ideal anti-backlash nut would have unlimited axial stiffness such that when a load is applied in either direction, no discernible movement would occur due to the elasticity of the nut itself. The anti-backlash aspect of the nut is only as good as the stiffness of the nut which has been created. Our U.S. Pat. No. 5,732,596 was directed to this problem and solved the problem by a high friction elastomeric means, such as a washer, provided between faces formed on each nut half normal to the screw to create high frictional contact between the high friction elastomeric means and the faces to limit relative rotational movement between the nut halves when they are in the closed abutting position and to permit the nut halves to translate along the rotational screw.
In solving this problem, another problem was created related to the compression of the elastomeric means, i.e. the washer. The washer does not behave in a uniform manner in certain instances of compression. The washer in compression follows a certain yield curve and then jumps to a second yield curve. The jump from one curve to the other leads to inconsistent results. It is the intent of this invention to solve such a problem.